This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Apoptosis, or programmed cell death, is a biochemical pathway critical to normal development and tissue homeostasis in multi-cellular organisms. Dysfunction of apoptosis results in development of human cancer and other diseases. The Bcl-2 family of proteins constitutes a crucial checkpoint in the apoptosis pathway. Bax is one of the two key proteins (Bax and Bak) of the Bcl-2 family that control the mitochondrion-mediated cell death pathway. In normal healthy cells, Bax locates in the cytosol and is maintained in the inactive state. Upon apoptotic stimulation, Bax undergoes conformational changes, and migrates from the cytosol to mitochondria. By insertion of its hydrophobic C-terminal end to the mitochondrial outer membrane, Bax disrupts mitochondrial membrane potential, and induces the release of pro-apoptotic proteins from mitochondria, which in turn triggers apoptosis. What maintains Bax in its inactive conformation in the cytosol of healthy cells, and what factors trigger the conformational changes of Bax upon apoptotic stimulation remain unclear. Our preliminary data showed that in addition to the majority of monomeric Bax, a small portion of Bax was associated with high molecular weight protein complexes in the cytosol of healthy cells. We hypothesize that the proteins that associate with Bax in healthy or apoptotic cells hold the key to Bax activation in apoptosis.